(or how to stay alive in severe turbulence)
Load Factor Limitations
We saw from the previous discussion that our ability to vertically change the direction of the airplane is solely dependent on how much excess angle of attack we have, or how far away from the critical angle of attack we are. At a slow cruise speed, we'd have enough excess angle of attack to pull 2 Gs, possibly more. As we speed up, we can pull even more Gs before the airplane stalls. Theoretically the possibilites would be limitless. There are only two problems, though.
Problem #1: Humans have trouble staying conscious at high G forces.
Problem #2: Airplanes have trouble staying together at high G forces.
I think Problem #2 is the main concern for us. If we pass out, most likely we will relax back pressure which would reduce load factor and hopefully allow us to regain consciousness again. If the airplane divides itself into more than one piece, load factor will definitely be reduced, along with our ability to safely land the airplane. If you look in Chapter 2 of most airplane flight manuals, you will find the operating limitations imposed upon that airplane. One of the limitations is the maximum certified load limits. For the Diamond Katana, the max load limit is 4.4 Gs positive. If we exceed that force, then we are in danger of turning one airplane into two or more (none of which would be capable of normal flight). So, it is definitely in our best interest to not exceed the maximum load limit. Normally this wouldn't be a problem, just avoid pulling back too hard on the stick. But what about situations beyond our control, like unintentional flight into severe turbulence?
Ladies and Gentleman, I present to you: Maneuvering Speed!!! What is maneuvering speed? Well, we talked about how load factor affects stall speed. An increased load factor essentially raises the stall speed of the airplane because it puts the airplane closer to the critical angle of attack. Let's look at some numbers for the Diamond Katana. In 1 G flight, at max gross weight, the indicated stall speed (with flaps up) is 42 kts. At 1.4 Gs, the stall speed increases to 55 kts. At 2 Gs, the airplane will now stall at 68 kts. Let's say we are flying along at 68 kts. If we were to start increasing the load factor, our stall speed would increase until it exceeded our current speed (68 kts), then the load factor would drop because the critical angle of attack was exceeded (which reduces lift). This would occur just as the load factor was reaching 2 Gs, and the airplane stalls before exceeding 2 Gs. Not only is there a stall speed for every load factor, but the reverse is also true: there is a maximum attainable load factor for every stall speed. In other words, for a stall speed of 55 kts, the maximum load factor is 1.4 Gs. This means that if you are flying at 55 kts, the airplane won't be able to exceed a load factor of 1.4, it will stall if you try. Do you see where I'm going with this? What if we knew the stall speed of the airplane at a load factor of 4.4 Gs, which is our positive load limit? For the Katana, that speed is 106 kts. This means that when flying at 106 kts (if at max gross weight), the airplane will stall as it reaches a load factor of 4.4 Gs. If we stay below 106 kts, we will never exceed 4.4 Gs, the airplane will stall before the load factor gets that high. So, for the Katana, the maneuvering speed is 106 kts.
Effect of Weight on Stall Speeds
Notice in the previous paragraph I stated that the Katana's maneuvering speed is 106 kts...if at max gross weight. We've pretty well established that an increased load factor causes the stall speed to increase. We have to produce more lift to compensate for the extra "weight", and we produce that extra lift by increasing angle of attack, which brings us closer to the critical angle of attack. Consequently, as you reduce load factor, the amount of lift that we need decreases, so we can decrease our angle of attack, and our stall speed is reduced. We've assumed that the actual weight of the airplane has stayed the same, and that an increase in load factor is the reason that we need to produce extra lift. What if we simply change the actual weight of the airplane? In other words, what if we land somewhere and drop off a few passengers? We are reducing the total weight of the airplane, which means that the wings don't have to produce as much lift, so our angle of attack is lower. So, in the same way that load factor changes our stall speed, the weight of the airplane also changes stall speed. Any time that you see published stall speeds, those are the stall speeds of the airplane when at maximum gross weight. If the airplane is at a lower weight, then the stall speeds will be lower. This is very important because maneuvering speed is a stall speed, and it will drop as the weight is reduced. In a Katana, for example, if you are lighter than max gross weight and flying at 105 kts, even though you are below maneuvering speed, it is still possible to exceed the max load limit of 4.4 Gs. So just remember, anytime you are at less than max gross, your maneuvering speed will be lower than that published in the airplane's flight manual.
If you like math you can figure out how the maneuvering speed will change when weight changes. The new maneuvering speed will equal the published maneuvering speed times the square root of the current weight divided by the max gross weight.
New Va = Old Va * Sqrt( Current Weight / Gross Weight )
Example: You are flying the Katana by yourself and at half tanks, the current gross weight is 1470. You're about to do some steep turns but you want to be sure that you won't exceed your positive load limit of 4.4 Gs. Let's calculate the maneuvering speed at your current gross weight.
New Va = 106 * Sqrt( 1470/1764 )
New Va = 97
So, in order to keep from exceeding your max load limits, you now need to stay below 97 kts.